As a method replacing a conventional radiography, there is known a radiation image recording and reproducing method utilizing stimulable phosphor, as described in JP-A 55-12145 (herein, the term "JP-A" means an unexamined and published Japanese Patent Application). In the method, a radiation image conversion panel (in other words, an image storage phosphor sheet) comprising a stimulable phosphor is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation having passed through an object or having radiated from an object, sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission), photoelectrically detecting the emitted light to obtain electric signals, and reproducing the radiation image of the object as a visible image from the electric signals. The panel having been read out is subjected to image-erasing and prepared for the next photographing cycle. Thus, the radiation image conversion panel can be used repeatedly.
In the radiation image recording and reproducing methods described above, radiation image is advantageously obtained with a sufficient amount of information by applying radiation to an object at a considerably smaller dose, as compared to conventional radiography employing a combination of a radiographic film and a radiographic intensifying screen. Further, in the conventional radiography, the radiographic film is consumed for every photographing; on the other hand, in this radiation image converting method, in which the radiation image conversion panel is employed repeatedly, is also advantageous in terms of conservation of resources and economic efficiency.
The stimulable phosphor, after being exposed to radiation, exhibits stimulated emission upon exposure to the stimulating ray. In practical use, phosphors are employed, which exhibit an emission within a wavelength region of 300 to 500 nm stimulated by stimulating light with wavelengths of 400 to 900 nm.
The radiation image conversion panel employed in the radiation image recording and reproducing method basically comprises a support and provided thereon a phosphor layer (stimulable phosphor layer), provided that, in cases where the phosphor layer is self-supporting, the support is not necessarily required. The stimulable phosphor layer comprises a stimulable phosphor dispersed in a binder. There is also known a stimulable phosphor layer, which is formed by vacuum evaporation or a sintering process, free from a binder and comprises an aggregated stimulable phosphor.
There is further known a radiation image conversion panel in which a polymeric material is contained in the openings among the aggregated stimulable phosphor. In these phosphor layers, the stimulable phosphor also exhibits stimulated emission upon exposure to the stimulating rays after absorbing radiation such as X-rays, so that the radiation having passed through an object or having been emitted from the object, is absorbed by the stimulable phosphor layer of the radiation image conversion panel, in proportion to the radiation amount and a radiation image of the object is formed on the panel, as a storage image of radiation energy. The storage image can be released by irradiating the stimulating ray, as stimulating emission light, which is photoelectrically read and transformed into electric signals to form an image as the storage image of radiation energy.
On the surface of the stimulable phosphor layer (i.e., the surface which is not in contact with the support) is conventionally provided a protective layer comprising a polymeric film or an evaporated inorganic membrane to protect the phosphor layer from chemical deterioration and physical shock.
Examples of the stimulable phosphor used in the radiation image conversion panel include a rare earth activated alkaline earth metal fluorohalide phosphor represented by the formula of (Ba.sub.1-x,M.sup.2+.sub.x)FX:yA, as described in JP-A 55-12145, in which M.sup.2+ is at least one of Mg, Ca, Sr, Zn and Cd; X is at least one of Cl, Br and I; A is at least one of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er; x and y are numbers meeting the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2; and the phosphor may contain the following additives.
In the above-described phosphors, there has not been paid any attention with respect to a rare earth metal in the composition of the surface or central portion of the stimulable phosphor and in particular, to the content of Eu. It was found by the present inventors that in cases when the composition in the surface portion of the stimulable phosphor is the same as that of the central portion, the level of lattice defects in the surface portion is greatly enhanced and particularly in the case of halide phosphors, instantaneous emission upon exposure to X-rays in the phosphor surface increases, resulting in an increase of an afterglow value of the instantaneous emission. In this case, when reading a radiation image stored in the stimulable phosphor after exposure to X-rays from a radiation image conversion panel, the instantaneous emission afterglow value is added, as a noise component, to read-out signals, leading to lowering of the S/N ratio.
Furthermore, in the case when the afterglow value of the instantaneous emission becomes greater, problems are produced such that high-speed photographing by the panel becomes impossible or becomes more sensitive to unevenness of the X-ray dosage produced at an X-ray source.